1. Title of the Invention
This invention relates to a method of producing biaxially oriented tubular polyetheretherketone films.
2. Description of the Prior Art
In recent years, the range of use of plastics films has been increasing and, at present, high-level performance characteristics are required of plastics films. Among others, heat resistance is required, particularly when they are used as spacecraft or aircraft materials or high-grade electric insulator materials. It is not going too far to say that, for the present, polyimide films are the only films that can meet such requirements. However, since polyimide films are produced by the solution casting method, commercial production of said films is in a very unfavorable competitive position in that large-scale production equipment is required, the production process is complicated, productivity is very low and, as a result, the cost of film production is high.
Under these circumstances, the present inventors attempted to provide a method of producing polyetheretherketone films having heat resistance, good optical properties and good and balanced lengthwise and breadthwise mechanical properties at low cost and in a highly efficient manner by the extrusion method (simultaneous biaxial drawing in the tubular form).
It is well known that when biaxially oriented, thermoplastic resin films generally acquire improved mechanical and optical properties and improved heat resistance. For the purpose of biaxial orientation, there are known the tenter frame process and the tubular process. In the tenter frame process, the successive drawing technique is generally employed which comprises heating an unoriented flat film to an appropriate drawing temperature and then drawing it in the longitudinal direction making use of rolls and then in the breadthwise direction making use of chucks. Complicated and large-scaled equipment is required and chucking directly means a loss of film (chucked portions are sacrificed). As a result, the efficiency of film production is low and the product films unavoidably exhibit directional differences in physical properties. On the other hand, the tubular process comprises introducing an unoriented film into the space between a feed roll pair and a pull roll pair, reheating the film externally to a temperature appropriate for drawing and allowing the film or bubble to expand and be stretched by means of the internal pressure. The equipment required is simple and small-sized. Film loss such as caused by chucking in the tenter frame process is not incurred at all. The process is thus characterized by very good productivity and product films have balanced lengthwise and breadthwise mechanical properties.
The polyetheretherketone resin is a crystalline resin. When crystallization progresses and crystallinity becomes excessively high, it becomes hard and brittle and, further, grey and opaque. In unoriented film form, it cannot be used in the practice and its mechanical properties cannot be improved by drawing. Therefore, it is necessary to improve polyetheretherketone films which respect to their mechanical properties such as film strength and optical characteristics by keeping their crystallinity at a low level and subjecting them to biaxial orientation. However, the mode of practice of the tubular process which is in general use, in which cooling of molten resin as it is extruded through a circular die is effected by means of a gaseous medium such as air, gives hard and brittle films, which are broken when submitted to feed rolls. Such films thus cannot undergo the subsequent drawing. It can safely be said that there is no precedent for the production of biaxially oriented polyetheretherketone films by the tubular process on a commercial scale.
It was the technical task given to the present inventors in such circumstances to make it possible to obtain polyetheretherketone films having high heat resistance, good optical properties and high and directionally balanced strength by simultaneous biaxial orientation by the tubular process which uses a relatively small equipment as compared with the tenter frame process and gives good operation efficiency and productivity. The inventors conducted intensive investigations in search of such conditions as to enable such tubular-form simultaneous biaxial orientation with the polyetheretherketone resin and, as a result, they found the method of the present invention.
Accordingly, it is an object of the invention to provide a method of producing polyetheretherketone films biaxially oriented by the tubular process.